1. Field of the Invention
The invention relates generally to plumb bobs for use in construction work and the like.
2. Discussion of Related Art
Plumb bobs are used by surveyors, engineers and artisans in construction work for establishing straight vertical lines. A plumb bob includes a body and a cord. The cord has a free end and an opposite end connected to the body. The cord is stored on a take-up spool. To use the plumb bob, the cord is unwound from the spool to an extended position and is suspended from the free end, such that the body hangs from the opposite end of the cord. Thus, the cord defines a straight vertical line between the free end and the opposite end connected to the body. In one arrangement disclosed in U.S. Pat. Nos. 2,492,609, 3,442,025, 3,478,438 and 4,459,761, the spool is supported separate from the body, and the cord at the free end is wound around the spool for storage and at the opposite end is connected to the body. In another arrangement disclosed in U.S. Pat. Nos. 2,714,766, 3,027,977 and 3,064,356, the spool is mounted on the body, such that the cord at the end opposite the free end is wound around the spool and is thereby connected to the body.
One type of plumb bob, commonly referred to as a retractable or automatic plumb bob, includes a spring such as a coil spring which exerts a biasing force on the spool and thereby causes the spool to rotate and retract the cord from the extended position to the storage position. A braking or locking mechanism is selectively operable to prevent the cord from being retracted onto the spool. Typically, the locking mechanism operates directly against the spool to prevent the spool from rotating under the biasing force of the coil spring. In one arrangement disclosed in U.S. Pat. No. 3,027,977, the locking mechanism operates against the cord to prevent the cord from being retracted onto the spool. The locking mechanism disclosed in U.S. Pat. No. 3,027,977 includes a pin perpendicular to the cord and having a pair of washers mounted thereon. The cord extends through a space defined between the washers, and the pin is biased toward a clamping position wherein the cord is clamped between the washers and is thereby prevented from moving relative to the spool. When the pin is moved from the clamping position by the user pushing a plunger, the cord is permitted to move for winding or unwinding relative to the spool.